1. Field of Invention
The invention relates in general to methods for examining the eye and specifically to the use of an electro-optical ophthalmoscope for imaging and psychophysical applications.
2. Description of Prior Art
The related U.S. Pat. No. 5,568,208, "Modified scanning laser ophthalmoscope for psychophysical applications", issued Oct. 22, 1996, discloses a scanning laser ophthalmoscope that is modified in order to know and control an important variable in psychophysical testing of the eye with the scanning laser ophthalmoscope, i.e. the entrance location of the Maxwellian view illumination or pivot point of the illuminating system of the scanning laser ophthalmoscope. For some psychophysical applications both the retinal location that is being tested and the entrance location of the Maxwellian view in the eye used for this testing have to be known, maintained or systematically varied according to an algorithmic design. In the prior art, this entrance location is determined with the help of a second imaging device that is attached to the scanning laser ophthalmoscope in a fixed position. A beamsplitter is usually employed to obtain a better frontal observation of the anterior segment of the eye. To acquire reference video images of the anterior segment prior to actual testing with this second imaging device, it is sufficient to focus on the iris, for example when the pivot point of the Maxwellian view, seen as a low intensity backscatter, is also in focus in different places on the anterior surface of the lens and iris. A harrier filter attached to the second imaging device and a wide angle third wavelength external light source facilitate the acquisition of reference images that contain supplementary anterior segment fiducial landmarks including Purkinje images of the light source, but do not contain any spurious reflexes from the scanning laser ophthalmoscope illuminating sources during testing. A two-dimensional normalized gray scale correlation or equivalent algorithm localizes unambiguously selected fiducial landmarks in the continuous stream of video images of the anterior segment of the eye, and calculates any displacement of the anterior segment relative to the pivot point of the Maxwellian view. Furthermore, a feedback loop, steering the positioning motors of the scanning laser ophthalmoscope, can then keep the pivot point of the Maxwellian view in the desired location within the anatomical pupil of the eye. This utility is highly beneficial. For example, when performing automated microperimetry on an extended area of the retina with the scanning laser ophthalmoscope, the pivot point of the Maxwellian view can be stabilized within the confines of the anatomical pupil, increasing the efficiency of the algorithms involved. Another psychophysical application that needs to know, maintain and systematically vary the entrance location of the Maxwellian view illumination is the measurement of Stiles-Crawford parameters or photopigment concentration for a specific retinal location. Two important limitations exist when reducing to practice the utility to control simultaneously the entrance location of the Maxwellian view and the retinal location being tested. First, variations in pupil size cannot be measured easily. These variations can also influence the efficiency and reliability of the two dimensional gray-scale correlation algorithms or their equivalent, since a change in pupil diameter is likely to change the aspect and position of some fiducial landmarks in the images of the anterior segment of the eye as well. Second, in order to adjust, maintain or systematically vary the entrance location of the Maxwellian view, it is necessary to move the scanning laser ophthalmoscope and subject relative to each other. This necessity increases the complexity of the equipment.